[vortex] causes 3c905c jitter ???

Donald Becker becker@scyld.com
Tue Dec 18 17:41:01 2001

On Tue, 18 Dec 2001, Stefan Doehla wrote:

> I have here a realtime application with RTLinux. It seemed to me that 
> some PCI cards vblock from time to time the PCI bus. E.g. video cards 
> tend to use the pci_retry feature of the PCI spec -> PCI bus is locked 
> for some microseconds. This problem was solved with a downgrade to XFree 
> 3.3.6.

This is a know problem with video cards.  Since a few percent difference
is considered a major advantage in the gaming market, the hardware tends
to cheat on the specs and be bandwidth hogs.  NICs are a much less
agressive market -- people are more realistic about performance and

> To my problem: XForwarding and some other network stuff (which should be 
> used later in conjunction with the realtime programs) causes also 
> jitter. It seems like the softwareside is okay.
> It looks like only outgoing traffic causes problems ...
> Can the 3com networkcard lock the PCI bus for some microseconds? Uses it 
> a fifo that's not checked before writing (due to performance gains)?

What chip, and what driver version?

You can change various settings to limit the PCI burst size, but I don't
know what you are seeing that's a problem.

Donald Becker				becker@scyld.com
Scyld Computing Corporation		http://www.scyld.com
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